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SubjectRe: [PATCH 1/2] usb: gadget: udc-core: independent registration of gadgets and gadget drivers
Hi Alan,

On Thu, Jan 29, 2015 at 5:56 PM, Alan Stern <> wrote:
> On Thu, 29 Jan 2015, Ruslan Bilovol wrote:
>> Change behavior during registration of gadgets and
>> gadget drivers in udc-core. Instead of previous
>> approach when for successful probe of usb gadget driver
>> at least one usb gadget should be already registered
>> use another one where gadget drivers and gadgets
>> can be registered in udc-core independently.
>> Independent registration of gadgets and gadget drivers
>> is useful for built-in into kernel gadget and gadget
>> driver case - because it's possible that gadget is
>> really probed only on late_init stage (due to deferred
>> probe) whereas gadget driver's probe is silently failed
>> on module_init stage due to no any UDC added.
>> Also it is useful for modules case - now there is no
>> difference what module to insert first: gadget module
>> or gadget driver one.
> It's possible to do all this much more simply. In fact, I posted a
> patch some time ago to do exactly this (but I can't find a copy of it
> anywhere).

Unfortunately I didn't find your patch.

>> Signed-off-by: Ruslan Bilovol <>
>> ---
>> drivers/usb/gadget/udc/udc-core.c | 113 +++++++++++++++++++++++++++++++++++---
>> 1 file changed, 105 insertions(+), 8 deletions(-)
>> diff --git a/drivers/usb/gadget/udc/udc-core.c b/drivers/usb/gadget/udc/udc-core.c
>> index e31d574..4c9412b 100644
>> --- a/drivers/usb/gadget/udc/udc-core.c
>> +++ b/drivers/usb/gadget/udc/udc-core.c
>> @@ -43,13 +43,23 @@ struct usb_udc {
>> struct usb_gadget_driver *driver;
>> struct usb_gadget *gadget;
>> struct device dev;
>> + bool bind_by_name;
>> + struct list_head list;
>> +};
>> +
>> +struct pending_gadget_driver {
>> + struct usb_gadget_driver *driver;
>> + char *udc_name;
>> struct list_head list;
>> };
> You don't need all this stuff. What's the point of keeping track of
> names? If there are any unbound gadget drivers pending, a newly
> registered UDC should bind to the first one available.

It's because gadget driver may be bound to usb_gadget in two ways:
- standard way - in this case any available udc will be picked up
- by name of udc, in this case only matching udc will be picked up

Main feature of my path is not only deferred binding of gadget driver,
but also possibility to register/unregister udc at any time.
This is useful for user who can load, for example, udc module
if needed and unload it safely, not touching gadget driver.
Another example is USB device controllers that consist of pair of
HS+SS controllers, each one having its own udc driver. In this case
it's possible to switch SS/HS by registering/unregistering corresponding
udc and not touching gadget driver.

I did all of this inside of udc-core because it looks like to be best place for
udc <-> gadget driver housekeeping. Also it is verified on lot of combinations
of udc and gadget drivers that can be built-in or build as modules

Best regards,

> Just add a "pending" list_head into the usb_gadget_driver structure and
> forget about all the rest. (Or try to find my patch in the mailing
> list archives somehow see if you think it needs to be changed.)
> Alan Stern

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